An image forming apparatus such as a copier, facsimile, and printer may produce an image-recorded sheet by fixing an image on a recording sheet. Such fixing may be conducted by applying heat and pressure to the recording sheet having an un-fixed image thereon.
Specifically, an image forming apparatus may include a fixing unit, to which a recording sheet having an un-fixed image thereon is transported to fix the un-fixed image on the recording sheet.
For example, the fixing unit may include a heating roller and a pressure roller to fix the un-fixed image on the recording sheet. The heating roller may apply heat to toner particles (or developing agent) included in the un-fixed image to melt toner particles. The melted toner particles may permeate into the recording sheet with an effect of the heating roller and pressure roller. With such fixing process, the fixing unit may fix the un-fixed image on the recording sheet. In general, such fixing unit may include a cleaning member to clean the heating roller, for example.
Conventionally, an image forming apparatus may include a photoconductor or an intermediate transfer belt, which may carry a toner image thereon. In such image forming apparatus, the toner image may be electrostatically transferred to a sheet from the photoconductor or intermediate transfer belt, and then the sheet may be transported to a fixing unit, in which the toner image may be fixed on the sheet to produce an image-recorded sheet.
In such a fixing unit, a sheet may enter and leave a fixing nip, defined between a heating roller and a pressure roller to fix the un-fixed image on the sheet.
Specifically, a front edge portion of sheet may enter and leave the fixing nip first, and a rear edge portion sheet may enter and leave the fixing nip last.
During such a fixing process, a rotational speed of heating roller or pressure roller may vary or fluctuate when the sheet enters and leaves the fixing nip.
In such a conventional fixing process, even if a heavy sheet (e.g., heavy paper), which may vary a rotational speed of rollers relatively greatly, enters and leaves the fixing nip, a rotational speed change of such rollers may not affect an image quality to be produced on the sheet.
In such a conventional fixing process, a rotational speed fluctuation of rollers may not affect an image quality when a front edge portion of a sheet enters the fixing nip or a rear edge portion of the sheet leaves the fixing nip.
However, an image forming apparatus having a following configuration may produce a lower quality image when such rotational speed fluctuation of rollers may occur in a fixing unit.
FIG. 1 shows an image forming section of an image forming apparatus having a plurality of photoconductors in a tandem manner.
As shown in FIG. 1, such an image forming section may include an intermediate transfer belt 2, a drive roller 9, a registration roller 18, a secondary transfer roller 71, a counter roller 71a, and a drive motor Mb, for example.
The secondary transfer roller 71 and counter roller 71A may define a secondary transfer nip TN2 therebetween, to which a sheet P or heavy sheet HP may be transported from the registration roller 18. The drive motor Mb may drive a traveling movement of the intermediate transfer belt 2.
In this disclosure, the sheet P may include a plurality of types of sheets, and the heavy sheet HP may indicate a thicker sheet such as heavy paper. The sheet P or heavy sheet HP may be used in this disclosure, as required.
When the sheet P enters or leaves the secondary transfer nip TN2, a load fluctuation may occur in a transportation direction of sheet P at the secondary transfer nip TN2.
Such a load fluctuation may be transmitted to the drive roller 9 via the intermediate transfer belt 2.
The drive roller 9 has a shaft, which may be linked to a drive motor Mb via a link mechanism or speed reduction mechanism. The drive motor Mb may include a DC (direct current) motor, a pulse motor, or the like.
As shown in FIG. 1, the intermediate transfer belt 2 may be extended by the drive roller 9. The intermediate transfer belt 2 may travel in a given direction with a driving force of the drive roller 9, which may frictionally move the intermediate transfer belt 2.
The drive motor Mb and drive roller 9 may be used to control a traveling movement of the intermediate transfer belt 2 precisely.
If a load fluctuation occurring at the secondary transfer nip TN2 is not adjusted by the drive roller 9, such load fluctuation may cause a deviation of traveling amount of intermediate transfer belt 2 from a normal traveling amount although such a deviational amount may be of a tiny scale
Such a traveling amount deviation may be transmitted to a primary transfer nip TN1 defined by a photoconductor (e.g., photoconductor drum) and the intermediate transfer belt 2, and then a deviation of image transfer position from a normal position may occur at such primary transfer nip TN1.
Such a deviation of an image transfer position at the primary transfer nip TN1 may be termed as shock jitter, which may be a deviational movement of a tiny scale.
If such shock jitter may occur, an image quality to be produced on a recording sheet may be degraded.
Furthermore, a condition of the drive roller 9 and other driving force transmission mechanism, which controls a traveling movement of intermediate transfer belt 2, may also be affected by several factors.
For example, such factors may include a smaller scale slipping of intermediate transfer belt 2 due to load fluctuation, an elongation of intermediate transfer belt 2 due to load fluctuation, deformation of gears included in a link mechanism or speed reduction mechanism, and a driving force degradation of drive motor Mb due to load fluctuation.
Recently, some image forming apparatuses may have been employing a transfer-fixing configuration, which may conduct an image transfer and fixing process in a seamless manner. Such a configuration may be preferable from a viewpoint of miniaturization of an apparatus and improvement of sheet transportation reliability.
However, in such transfer-fixing method, a shock jitter may unfavorably become greater when a sheet enters and leaves a nip defined by transfer-fixing configuration.
If such greater shock jitter occurs, such shock jitter may affect an image quality to be produced on a sheet.
Specifically, a shock jitter may occur when a front edge portion or rear edge portion of sheet P may pass through a fixing nip for transfer-fixing configuration. Such shock jitter may unfavorably become greater when the sheet P is a heavy sheet (or heavy paper).
Such shock jitter may occur at the primary transfer nip TN1 when a load fluctuation occurring at the fixing nip in a transfer-fixing configuration is transmitted to a secondary transfer nip TN2, and then to the primary transfer nip TN1 via the intermediate transfer belt 2.
A fixing nip pressure in such transfer-fixing configuration may generally be set to a greater value than a secondary transfer nip pressure at the secondary transfer nip TN2 shown in FIG. 1, by which an effect of shock jitter may become relatively greater.
Accordingly, such relatively greater shock jitter may degrade an image quality to be produced on a sheet, wherein such degradation may be observed as “banding (e.g., unintended stripe-like image)” on a sheet.
FIGS. 2A and 2B show a schematic configuration for measuring a speed fluctuation at a nip portion such as secondary transfer nip by using the registration roller 18 and a measuring roller 50.
The measuring roller 50 may be provided in a position corresponding to a secondary transfer nip.
FIG. 2A shows a timing when a front edge portion of the sheet P enters the measuring roller 50, and FIG. 2B shows a timing when a rear edge portion of the sheet P leaves the measuring roller 50.
Based on actual testing results conducted with a configuration shown in FIGS. 2A and 2B, it has been learned that a speed fluctuation of a sheet transport speed at the measuring roller 50 may be observed when a front edge portion of the sheet P enters the measuring roller 50, and a rear edge portion of the sheet P leaves the measuring roller 50. Such a speed fluctuation may become greater if a heavy paper is used as the sheet P.
FIG. 3 shows an example chart explaining a measurement result of a speed fluctuation experiment conducted using a configuration shown in FIGS. 2A and 2B.
In FIG. 3, signals Pi and Po may respectively correspond to conditions shown in FIGS. 2A and 2B. As shown in FIG. 3, a heavy sheet (or heavy paper) may cause a relatively greater speed fluctuation when the heavy sheet passes through a nip portion, which is not favorable from a viewpoint of shock jitter.